The character known as Slayer was once a young Caucasian male that was presumably captured and experimented on by Dr. Jakob Silver and his associates, at their headquarters in Washington. After Jakob developed a powerful serum which he named the B-Virus, his subjects were injected with the substance and measured on their results. Several died, but Slayer and another did not. The other manifested into a vampire like creature and had to be destroyed, but Slayer developed superhuman strengths, and a supposed immunity to further exposure of the Virus.
Watching and observing his captors through his imprisonment, and not recollecting any of his past, Slayer escaped from the facility and murdered several of Silver's staff, before going into hiding. Angered by the loss of his valuable subject, Silver decided to unleash the Virus onto a small population, but within a few weeks the Virus spread nation wide. After he was found and treated by a group of survivors (Dr. Sarah Jones, Dr. Michael Roberts, and Jack Li) the group went underground and sought refuge in the City, a vast underground complex built in the threat of humanity's destruction.